Conventionally, as a ground structure of a circuit, the following structure is known.
Patent Literature 1 discloses a noise suppressing circuit structure. A noise suppressing circuit unit thereof includes a power supply unit, a noise filter unit, and a terminal unit. The terminal unit includes a power-supply line terminal, an external frame ground terminal, a signal ground terminal, and an internal frame ground terminal. Housings of respective units are connected to each other with sheets of metal. Further, an output terminal of a ground potential of the power supply unit and the signal ground terminal of the terminal unit are connected to each other via a cable. Further, a ground terminal of the noise filter unit and the internal frame ground terminal of the terminal unit are connected to each other.
Patent Literature 2 discloses a connection method for a control board using a high frequency signal of equal to or higher than 1 MHz and a low frequency signal. According to this technique, a signal ground is connected to a frame ground at a single point. Along with this connection, the signal ground is connected to the frame ground at multiple points via a capacitor.